Field of the Invention
The present invention relates to a mode converter and a method for manufacturing the same, and more particularly, to a technique that is used for a mode converter for millimeter-wave band communication.
Priority is claimed on Japanese Patent Application No. 2013-029486, filed Feb. 18, 2013, the content of which is incorporated herein by reference.
Description of the Related Art
In recent years, high-capacity communication which uses a millimeter-wave band and has a high transmission speed of several gigabits per second [Gbps] has been proposed and a portion thereof has been achieved. In particular, the importance of a wireless communication device which operates in a band of 60 [GHz] has increased. In Japan, it is possible to use a wide frequency band of 59 [GHz] to 66 [GHz], without a license. Therefore, the wireless communication device is expected to be spread to the household sector. Therefore, the most urgent task is to achieve an inexpensive and small millimeter-wave communication module.
R. Suga, et al., “Cost-Effective 60-GHz Antenna-Package with End-Fire Radiation from Open-Ended Post-Wall Waveguide for Wireless File-Transfer System,” 2010 IEEE MTT-S International Microwave Symposium, pp. 449-452 (hereinafter referred to as NPL 1) and Japanese Unexamined Patent Application, First Publication No. 2011-109438 (hereinafter referred to as PTL 1) disclose a millimeter-wave module using a waveguide (post-wall waveguide antenna: PWA) which is formed by a printed circuit board as a technique for achieving a small and inexpensive millimeter-wave communication module.
As shown in FIGS. 1 to 7 of PTL 1, in the technique disclosed in PTL 1, the side wall (metal wall) of a waveguide according to the related art is replaced with a through hole group (post group) of the printed circuit board. As shown in FIGS. 1 to 7 of PTL 1, a wireless communication IC (CMOS-IC) is mounted on the PWA. A millimeter-wave signal which is output from an IC (which is referred to as a semiconductor chip 4 in the specification of PTL 1; which holds for the following description) by a method, such as wire bonding or bump connection, is transmitted through a transmission line (which is referred to as a line 24 such as a microstrip, a coplanar strip, or a strip) formed by a plane circuit once. Then, the millimeter-wave signal is finally transmitted to a waveguide structure portion (which is referred to as a waveguide 2) through a plane circuit/waveguide conversion structure (which is referred to as a central conductor 23).
FIG. 28 shows an example of the structure of a mode converter (converter) according to the related art. As shown in FIG. 28, in a mode converter 510, a waveguide 502 includes an opening portion 525 which is provided in a forward end surface (the right side of FIG. 28) and from which radio waves are radiated. The waveguide 502 includes a plurality of post (column) walls 520 and upper and lower grounding conductor layers (cooper films) 521 and 522. A pin (plane circuit/waveguide converter) 523 is inserted as a feed portion into the waveguide 502. A millimeter-wave signal which is introduced from a transmission line 524 to the pin 523 is radiated as electromagnetic waves from the opening portion 525 provided on the front side of the waveguide 502. The mode converter 510 is formed by laminating a plurality of substrates 528A, 528B, and 528C. The pin 523 is formed by laminating the substrates 528B and 528C in which vias are formed in advance and by filling the vias with a conductive material.
In the design of a general high-frequency circuit, when a circuit A and a circuit B are connected to each other, it is necessary to perform impedance matching. This means that a signal is transmitted without being reflected at a connection point between the circuit A and the circuit B. That is, a signal needs to be transmitted without being reflected at a connection point between a plane circuit/transmission line as the circuit A and the waveguide as the circuit B.
In the structure shown in FIG. 28, in a predetermined frequency band, the length of the pin 523 is adjusted to a predetermined value to adjust impedance. As a result, signal transmission with low reflection loss is achieved. In addition, a method for optimizing the distance between the pin 523 and the grounding conductor layer 522 is considered as one of impedance matching means.
However, in a method for manufacturing the converter according to the related art, the vias are formed in advance in a plurality of substrates whose thickness has been determined and the plurality of substrates are laminated to manufacture the pin. Therefore, the length of the pin depends on the thickness of the substrates and is set to only a discrete value. As a result, it is difficult to set the length of the pin to a desired value. In addition, it is difficult to set the thickness of each of the substrates to be laminated and the thickness of each substrate depends on the availability of a material. Therefore, it is difficult to obtain a pin with an optimal length.
When reflection loss is reduced by means other than the adjustment of the length of the pin, the means are likely to be a band limiting factor which causes a reduction in an available band. Therefore, the means are not the best method.
In addition, for example, the following undesirable situations occur: transmission loss caused by an adhesive which is used to attach the substrates; a variation in transmission characteristics caused by the material forming each layer; and the positional deviation between the substrates when the substrates are laminated.
The invention has been made in view of the above-mentioned problems and an object of the invention is to provide a method for manufacturing a mode converter which achieves signal transmission with low reflection loss and facilitates impedance matching.